Apparatus for grinding the surfaces of small objects



H. F. FRUTH v 2,405,417 APPARATUS FOR GRINDING- THE SURFACES OF SMALL OBJECTS Filed July 9, 1943 2 Sheets-Sheet 1 6, 1946. H. FQ FRUTH APPARATUS FOR GRINDING THE SURFAGES OF SMALL OBJECTS 2 Sheets-Sheet 2 Filed July 9, 1943 INVENTOR.

Patented Aug. 6, 1946 APPARATUS FOR GRINDING THE SURFACES OF SMALL OBJECTS Hal F. Fruth, Chicago, Ill., assignor to Galvin Manufacturing Corporation, Chicago, 111., a corporation of Illinois Application July 9, 1943, Serial No. 493,964

. 3 Claims.

The present invention relates to apparatus for grinding the surfaces of small objects and more particularly to improved apparatus for grinding the faces of piezoelectric-crystals.

In the manufacture of certain small flat parts or objects, it is frequently necessary to grind the parts precisely to predetermined thicknesses. Thus, in the manufacture of piezoelectric quartz crystals adapted for use in crystal microphones, radio transmitting and receiving systems and the like, crystal blanks which have been cut from the crystal stock, are ground to the desiredrectangular dimensions. The grinding operations are carried out in several steps, among the first of which is that of pressure grinding the faces of each blank in order to reduce the blank aping the pressure grinding operation, the less is the time required to finish grind the crystal faces to produce the exact desired resonant frequency characteristic for each crystal.

It is an object of the present invention, therefore, to provide improved and exceedingly simple apparatus for rapidly and accurately grinding the fiat surfaces of small objects.

It is another and more specific object of the invention to provide apparatus for pressure grinding the faces of piezoelectric crystals in order rapidly and accurately to reduce the crystals to a desired thickness.

In accordance with another object of the invention, provisions are made for successively and repeatedly sweeping the faces of a plurality of commonlyrsupported crystals across a continuously moving abrasive surface, and facilities are provided for adjustably positioning the crystal faces relative to the abrasive surface in order to adjust the depth of each cut or to provide for successive cuts of the same depth.

According to still another object of the invention, improved facilities are provided for indiabrading operation is started, thereby to provide for a cut of uniform depth in each crystal face during each abrading operation.

In accordance with a further object of the invention, the apparatus is so arranged that the position of each crystal may be automatically changed to shift the line of cut during each face abrading operation, thereby to prevent the accentuation of scratches which may be produced in the crystal faces during a given abrading operation.

According to another and more specific object of the invention, facilities are provided for selectively and continuously rotating each crystal about an axis which bisects and is normal to its faces in order to shift the line of cut during successive abrading operations.

It is still another object of the invention to provide improved facilities for detachably supporting the crystals within individual crystal holding pockets of the apparatus in a manner such that a solid crystal bearing surface is furnished which will minimize crystal breakage.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings, in which:

Fig. 1 is a side elevational view of improved pressure grinding apparatus characterized by a number of the features briefly referred to above;

Fig. 2 is a fragmentary detail view in section of certain of the parts embodied in the apparatus shown in Fig. 1;

Fig. 3 is a side sectionalview of a modified arrangement for rotatably supporting a plurality of objects so that they 'may be successively actuated into engagement with anv abrasive surface;

Fig. 4 is an elevated perspective view of the holding apparatus shown in Fig. 3;

Fig. 5 is a side View, partially in section, of an improved mechanism for adjustably positioning the movable abrasive surface relative to the faces of the objects supported by the apparatus shown in Figs. 3 and 4;

Fig. 6 is an end sectional view taken along the lines 6--6 of Fig. 5 illustrating further details of the adjustingmeans;

Fig. 7 is a partially schematic side elevational view of the mechanism which, is provided for driving certain parts of the assembly shown in Figs. 3 and 4; and

- Fig. 8 is a side sectional view of a modified object holding arrangement. 7 Referring to the drawings and more particushown.

3 larly to Figs. 1 and 2 thereof, the apparatus there illustrated comprises a heavy metal base In which is provided with facilities for slidably supporting a crystal carrying member H and for rotatably supporting an endless abrasive belt l2. More specifically, the crystal holding member ll comprises a steel bar of substantially square crosssection which is machined and ground to a uniform thickness throughout its length. This bar is slidably mounted upon the upper machined surface of the base l between two guide strip 7 i3 and M which are anchored to the base [0 at opposite ends thereof by means of assembly screws H5 and 6. The bar H is slotted transversely across it upper surface, and stop strips l! are wedged into the slots thus provided so that the upper edges thereof project slightly above the upper surface of the bar ll. Thus, a plurality of stops are provided at spaced-apart points along the length of the bar H which may be used to abut the edges 'of crystals 18 or other flat objects supported upon the upper surface of the bar. For the purpose of retaining the illustrated piezoelectric crystal blanks l8- face downward upon the upper surface of the bar "I I, this bar is provided with a central bore H) which is connected by means of a flexibl tube 20 to the inlet side of a vacuum pump 2i and communicates with apertures 22 drilled'downwardly from the upper surface of the bar. These openings are respectively disposed intermediate adjacent pairs of the stop strips H to underlie the lower faces of the crystals lBIsupported upon the bar H, so that during operation of the vacuum pump 2i, the atmospheric pressure acting upon th upper surface of each crystal will serve to maintain the crystal in tight frictional engagement with the upper surface of the bar. Thus, the crystals l8 may be firmly supported upon the upper surface of the bar ll during movement of this bar beneath the adjacent surface of the belt l2.

The facilities for rotatably supporting the abrasive belt 12 compris a, contact roller 23 and a backstand idler roller 24 which are respectively supported upon a drive shaft 25 and an idler' shaft to. More specifically, the driving or contact roller 23 is rigidly mounted upon the front end of the shaft 25 as viewed in Fig. 1 of the drawings, is formed of steel or other nonyieldable material and is provided with a straight contact surface across the width thereof. The shaft 25 is journaled in bearings 21 which are supported upon two spaced-apart upstanding bearing pedestals 28 anchored to the base [0. This shaft is provided at its rear end with a pulley 29 which is connected by a belt 30 to the driving pulley 3! of an electric motor 32 rigidly mounted upon the base It by means of mounting screws or bolts 33. The backstand for supporting the idler shaft 26 comprises an arm 34 which is axis mounted at 35 upon a bracket 35 carried by the base plate In. For the purpose of tensioning the belt I2 as looped about the two rollers 23 and 24, a coil spring 31 is provided which is anchored at one end to the upper end portion of the arm 34. At its opposite end, the spring 37 is anchored to an anchor pin 38 which is carried by a bracket piece 39 rigidly mounted upon the base I 0. Crystal dust and particles of the abrasive material which may be detached from the belt l2 are prevented from ac cumulating upon the faces of the crystals I8 by providing an air blast nozzle 40 which is fluid connected with a source of compressed air, not

The character of the granular abrasive material 4 which is bonded to the outer surface of the belt l2 will of course depend upon the type of material to be abraded, the desired speed of abrasion, and other factors. It has been found, however, that for the rough grinding of piezoelectric crystal blank formed of quartz, the abrasive granules may consist of No. 320 screen mesh silicon carbide. Any appropriate and conventional method of bonding the abrasive particles to the cloth backing of the belt may be utilized, but it is pointed out that the process employed should be of such character that the sharpest cutting face of each particle is presented to the crystal faces during rotation of the belt in a selected direction.

One of the problems involved in providing a, belt suitable for accurate and precision grinding, is that of joining the ends of an abrasively coated strip to form the desired endless loop. This may be accomplished by utilizing a cemented butt splice between the strip ends and backing the joint with a strip of wire cloth of small mesh and small strand diameter. The strip of wire cloth may be positioned to overlap both abutting end edges of the strip and then pressed into the cloth backing. When the joint is formed in this manner, the thickness thereof is no greater than the thickness of the belt itself and may in fact be thinner.

In utilizing the above-described apparatus to grind the faces of piezolectric quartz crystal blanks, for example, the belt l2, as looped about the contact roller 23 and the backstand roller 24, is driven by the motor 32 at a high rate of speed which may range from 2,000 to 3,000 surface feet per minute. In order to load the crystal holding bar 1!, this bar is withdrawn to the left from beneath the surface of the contact roller 23, following which crystals are disposed face downwardly along the upper surfaces of the bar with an edge of each crystal in abutting engagement with the projecting edge of one of the stop strips I 7. Assuming that the contact roller 23 is rotated by the motor 32 in a counterclockwise direction as viewed in Fig. 1 of the drawings, each crystal i8 is positioned to abut that stop strip H which is disposed to the right thereof, in order to prevent it from being [slid across the surface of the bar H as it is moved into abrading engagement with the abrasive surface of the belt l2. Either before or after the crystals have been thus position-ed upon the upper surface of the bar I], operation of the vacuum pump 2| may be initiated for the purpose of causing the crystals frictionelly to grip the upper surface of the bar.

After the bar II has been loaded with crystals, it may be slid between the guide strips 53 and It for the purpose of bringing the upwardly disposed faces thereof successively into engagement with the ever changing portion of the belt I2 which embraces the constantly changing contact segment of the roller 23. As each crystal is moved beneath the contact segment of the roller 23, it is ground or abraded by the outer abrasive surface of the belt l2. The depth of each cut is of course determined by the distance between the upper surface of the bar H and the lower abrasive surface of the belt, as well as by the initial thickness of the crystals. Inthis regard, it is pointed out that the gap separating the'upper supporting surface of the bar II and the lowermost surface segment of the belt I2 is so chosen that crystals of maximum thickness will not be out to a depth of .more' than .010

in .the manner just explained. As thus arranged, it may occur that during the initial abrading operation when the bar I! is first slid beneath the 'contact roller 23, only the upwardly disposed faces of those crystals of maximum thickness will be ground. After the initial cut or pass is taken, a fiat shim strip havin a measured thickness equal to the desired depth of the second out is interposed between the guide strips l3 and I4 beneath the lower surface of the bar H and the supporting surface of the base l0. Preferably, the thickness of this shim strip should not exceed .010 inch, thereby to limit the cut produced in the upwardly disposed crystal faces to a corresponding depth. By progressively increasing the thickness of the shim strip which is interposed between the lower surface of the bar I I and the upper bearing surface of the base between successive cut of the crystal faces, the crystals may obviously be reduced to any desired thickness. After each cut or pass, the crystal blanks may be rotated through an angle of ninety degrees with respect to the supporting bar I I, thereby to shift the line of out across their faces by a corresponding angle and thus prevent the accentuation or deepening of scratches which may be produced thereon during a particular abrading operation. It will also be understood that after the upwardly disposed facesfof' the crystals have been ground to the desired degree of flatness, they may be turned overso that the opposite faces are ground during succeeding passes of the bar I! between the guide strips l3 and I4; If this procedure is followed,'the crystals as finally reduced to the desired thickness will necessarily he possessed of faces which are absolutely flat andwill have uniform thickness over the surface area'sthere'of.

In the modified embodiment of the apparatus illustrated in Figs. 3 to '7, inclusive, of the draw ings, improved facilities are provided for individually adjusting the crystal faces relative to the abrasive belt l2a, for selectively rotating the crystal holders in order continuously to adjust or shift the line of cut across the crystal faces, and for changing the position of the contact roller in order to obtain the desired depth of cut across the crystal faces without resorting to shims or the like. Briefly considered, this modified apparatus comprises a stationary and rigidly mounted bearing member 4| having a flanged bearing portion 4la at its upper end, upon which all of the remaining parts of the holder assembly are rotatably supported. The assembly further comprises a pedestal member 42 which is rotatably journaled on the upper bearing surface of the member 4| and rigidly mounts a worm gear 43 adjacent its lower end. This worm gear is keyed to the shaft portion 4211 by means of one or more keys 44, and is supported upon a flanged assembly sleeve 45. More specifically, the sleeve 45 is threaded onto the lower threaded end of the shaft 420, and is provided with a flanged bearing portion 45a which 'interfits with the upper flanged bearing portion of the member 4|, thus providinga rigid assembly of the three parts 42, 43 and 45 which is r0- tatably mounted upon the upper end of the bearing member 4|. The assembly of the three enumerated parts is completed by means of assembly screws 46 which extend through openings in the sleeve 45 and are threaded into tapped holes provided in the worm gear 43. Upon its upper platform-like surface, the pedestal 42 supports a plate 41, which plate is secured to the upper surface of. the. pedestal. 42 by .meansuof assembly screws 48. The'plate 41 is provided, at-spaced points around the upper surface thereof, with bearing openings 49 within which holding bases 55 are journaled. Each holding base is provided with a shaft portion which extends through one of the openings 49 into engagement with the cut away surface of the pedestal 42 and carries a driving pinion 54 at its lower end. Further, each holding base 50 is provided with a central bore which is adapted to receive the shaft portion 52a of an object holder 52. In order to provide for vertical adjustment of each holder 52 relative to its associated holding base 50, each holding base is drilled and-tapped through the crown thereof to receive a set screw 53 which when threaded through the tapped'opening engages the shank portion 52a-of the holder 52. Eachobject holder is provided at its upper side with a recess or pocket 54 of such configuration that it will snugly embrace the edges of the flat objects which are to be ground.

For the purpose of imparting rotary movement to the holding bases 50 and the holders 52 supported thereon; a mechanism is provided for driving the pinions 5|- This mechanism comprises a ring gear 55 having internal teeth which mesh with-the teeth of the several pinions 5i and is arranged tobe driven by a worm gear 56 through a driving member 51. More particularly, the ring gear 55 is mounted upon the upper peripheral surface of the cup-shaped portion of the driving member 51 by means of screws 58, and the driving member 51 is journaled on the shaft portion 42a-of the pedestal member 42. The worm gear 56 iskeyed to the lower bearing portion of the driving-member 51" by means of one or more keys 59'and is rigidly mounted upon this ducing-a suction at the undersurface of each crystal. To this end, the two parts 4| and 42 are respectively provided with registering central bores 41b and42b, and the upper platform portion of the pedestal member'42 is drilled radially inward from" the peripheral edge thereof to provide a plurality of passages 420 which communicate with the central bore'42b. The passages 42 are closed at their outer ends by insert screws 5| which are screwed into the threaded end portions of the passages. They are so arranged that the end portion of each passage is-centrally intersected by the axis of rotation of one of the holding bases 55. Additional passages 42d drilled inwardly from the upper cut away-surface of the pedestal member '42 and respectively communicating with the passages-42c at the outer ends thereof are also provided in the pedestalmember 42. The passages 42d respectively communicate with passages 52b drilled axially throughthe holders 52. Thus, a system of passages is provided, whereby the central bore 41b is fluid connected to the mouth of each passage 52b disposed adjacent the undersurface of each crystal or other object I8 held in oneof the holders 52. The central bore Mb-may be conduit connected to'the inlet side of a vacuum pump of the character shown in Fig. l of the drawings for the purpose of creating a suction at the mouth of each of the passages 52b, whereby each-crystal or other object [8 may be forced by atmospheric pressure into engagement with the bottom-surface of the pocket 54 within which it isdisposed.-- In order to break-the-vacuum which may thus be produced at the mouth of each passage 52b, thereby to facilitate removal of the crystals I8 from their respective holders 52, a manually operable valve 62 is mounted upon the pedestal member 42 within the recessed portion 63 of the plate 41 so that the passage therethrough is in fluid connection with the central bore 421) of the member 42. Thus, an arrangement is provided whereby the crystals may be firmly gripped within the holding pockets 54 of the holders 52 and may be easily removed therefrom, as desired.

The driving mechanism for the relative movable parts of the holding assembly is schematically illustrated in Fig. 7 of the drawings.

Briefly considered, this mechanism comprises a worm 64 which meshes with the worm gear 43 and is connected by means of a shaft 65 to be driven by a motor 56 through a speed reducing gear box 61.

are utilized to impart rotation to the parts 42 and 4! of the holding assembly, thereby successively to move the holders 52 beneath the contact segment of the abrasive belt iZa. In

order selectively to rotate the holding bases relative to the supporting plate 41', or alternatively to hold these bases stationary, a driving connection is provided between the worm gear 56 and the shaft 65. This connection includes a driving worm 68 and the elements of a com- :v

bined clutch and speed changing device indicated generally at 69. More specifically, the worm 68 which meshes with the Worm gear 56 is mounted upon one end of a shaft 10 having a driving gear H mounted upon the opposite end thereof. The

driving the worm gear 56 in one direction when 5-? the shaft is rotating in a particular direction. The gear set M, on the other hand, comprises three intermeshed idler gears Ma, 1412 and Me, the first and last of which are arranged to mesh with the gears H and I2 to drive the worm gear .1.

5B in the opposite direction. The arrangement is such that when the worm gear 56 is driven through the gears of the gear set M, it is moved at the same speed and in the same direction as the worm gear 43. gear set 13, on the other hand, the worm gear 56 is rotated in a direction opposite the direction of rotation of the worm gear 43 and at a different speed. Suitable facilities are provided whereby the operating member H may be operated to any one of three different positions wherein no driving connection is provided between the shaft 55 and the worm gear 56; a connection is provided for rotating the worm gear 56 in one direction: and a connection is provided for rotating this gear in the opposite direction. The purpose of this arrangement will be more fully apparent from the following explanation in which the operation of the apparatus is specifically described.

Referring now more particularly to the arrangement cf the apparatus for raising and lowering the contact segment of the roller 23a relative to the top surface of the supporting plate 41, this apparatus is illustrated inFigs. 5 and 6 of the drawings as comprising twointerfitting U- The enumerated parts 64, 65, 66' and 61, in combination with the worm gear 43,

When driven through the iii) shaped supporting members '18 and 19 which are adjustably supported one within the other by means of screw shafts B8 and BI. The legs of the member 19 are provided with feet 192) which are rigidly clamped to the available supporting structure for the bearing member M by means of bolts 88. At their upper ends, the two shafts 80 and 8! respectively mount Worm gears 82 and 83 which commonly mesh with the teeth of a worm shaft 84. More particularly, the two shafts 80 and BI are suitably journaled in holes bored through the upper base wall We of th inverted U-shaped supporting member 19 in a manner such that axial movement of the shafts relative to the wall 15a is prevented. These shafts are reversely threaded at their lower ends and are screwed into tapped openings provided at opposite ends of the member 18, so that when the worm shaft 84 is rotated in one direction to reversely rotate the two worm gears 82 and 83, the member 18 is raised, and when the worm shaft 84 is rotated in the opposite direction, the member 18 is lowered. In this regard, it will be apparent by considering Fig. 6 of the drawings that the two members 18 and 19 are provided with bearing wall surfaces which prevent lateral or pivotal movement of the member 18 relative to the member 79. At their lower leg ends, the member 18 is provided with aligned bores which receive'bearings 85 and 86 within which the operating shaft 25a for the contact roller 23a is journaled. Suitable sleeves 81 abutting the end surfaces of the member 18 and set screw mounted upon the shaft 25a may be utilized to restrain the shaft against axial movement relative to its sup porting structure. At its left end, this shaft carries a driving pulley 29a which may be connected by means of a belt 35a to a driving motor in the exact manner illustrated in Fig. 1 of the drawings.

In considering the operation of the apparatus shown in Figs. 3 to "I, inclusive, of the drawings, it may be assumed that this apparatus is to be utilized in grinding the faces of piezoelectric quartz crystals. To this end, the crystals are individually supported within the pockets 54 of the holders 52, after which the valve 62 may be closed to permit the air within the connected passages Mb, 42?), 42c, 42d and 52b to be exhausted, whereby the crystals are firmly retained in engagement with the bottom walls of the pockets 54 within which they are respectively disposed. Either before or after the crystals are positioned within the pockets of the holders 52, the position of the contact roller 23a may be suitably established to providefor the desired depth of out across the crystal faces. This cut may be ascertained by micrometer measurements of the crystal thicknesses before the crystals are inserted within the receiving pockets of the holders 52. In order to set the Contact roller 23:; in the desired vertical position, the worm shaft 84 may be suitably actuated for the purpose of raising or lower ing the bearing support '58. In this regard, it will be understood that the operating end of the worm shaft is provided with a suitable actuating handle, and may also be equipped with a micrometer pointer and scale arrangement which is calibrated to indicate the distance between the lowermost contact surface of the belt in and the bottom wall surfaces of the holders 52 when these holders are engaged with the upper bearing surfaces of their respective holder bases 50.

After the position of the contact roller 23a has been established in the manner just explained thereby to move the crystals [8 through a closed circular orbit and thus bring the upper crystal surfaces successively into engagement with the contact portion of the belt l2a. At the end of each pass, i. e. after the supporting plate 4! has been rotated through one revolution so that the faces of all crystals supported by the holders 52 have been abraded, the worm shaft 84 may be readjusted through a predetermined angle for the purpose of producing a second cut in the crystal faces during the next succeeding revolution of the supporting plate 41. Thus, by continuous manual adjustment of the worm shaft 84 at the end of succeeding revolutions of the plate 41, the crystal faces may be ground to any desired depth for the purpose of producing any desired crystal thickness. Here again, the crystals may be turned over after one face of each crystal has been bringing the upper faces of the crystals of different thicknesses substantially into alignment, that is, into the same plane. This operation may be performed by loosening the set screws 53 and by utilizing micrometer shims inserted between the lower surface of each holder 52 and its associated holding base to establish the desired vertical position of the holder 52 in accordance with the particular thickness of the crystal which it is to hold during the grinding operation. After the position of a holder 52 is thus established, the associated set screw 53 may be tightened to retain the holder 52 in its elevated position. Thus, by providing a two-part, relatively adjust able crystal holding assembly, maximum flexibility is obtained in the matter of grinding crystals of different thicknesses.

The movement of the crystal faces which occurs during rotation of the supporting plate 4'! in the manner explained above, depends upon the setting of the operating member H as provided in the clutch and change speed mechanism 55. With this member occupying the position illustrated in Fig. '7 of the drawings, such that no driving connection is provided between the gears 72 and TI, the worm gear 55, the member 5? and the ring gear 55 are restrained against rotary movement. Accordingly, as the plate l! rotates, the pinions 5| carried by the base holders 55 are rotated Within the ring gear 55 to rotate the holders 52 relative to the plate 41. The direction of rotation of each holder relative to the plate 41 is of course determined by the direction of rotation of the plate il. Thus, if the supporting plate 41 is being rotated in a clockwise direction, as viewed from the top thereof, each holder 52 will be rotated in a counterclockwise direction relative to the plate.

If the operating member ll of the clutch and change speed mechanism 58 occupies a position such that the gears Ma and Me respectively enease the gears H and 72 such that a drive' connection is provided between the two shafts and 10 10, the driving member 5! and the ring gear 55 carried thereby are rotated in the same direction and at the same speed as the supporting plate All. Accordingly, no relative movement is produced between the ring gear 55 and the supporting plate 41, with the result that the pinions 5i are not rotated to revolve the holders 52 relative to the plate 41. In this case, the positions of the upwardly disposed crystal faces relative to the contact sector of the abrasive belt in are obviously not changed during rotation of the plate 41.

As a further alternative, if the operating member ll of the device 59 is shifted to the position wherein the gears 13a and 731) respectively engage the gears 'H and 12, a driving connection is established through which the ring gear 55 is rotated in a direction opposite that of the plate ll. In this case, the holders 52 are rotated at a relatively high rate of speed relative to the plate 41. It will thus be apparent that, depending upon the setting of the change speed and clutch device 69, the holders 52 may be held stationary with respect to the supportin plate ll or may be rotated with respect to this plate at either of two different speeds. The purpose of rotating the holders 52 with respect to the plate 41 is that of continuously changing the line of cut across the upwardly disposed surfaces of the crystals supported within the holders. Thus, the crystal faces are not only rotated to shift the positions thereof while out of engagement with the contact sector of the belt l2a, but are continuously rotated during the abrading thereof. As a result, scratche or grooves which may be formed in the crystal surfaces during arr-abrading operation are not accentuated-but, on the contrary, are obliterated'durin' a succeeding'abrading operation. In this regard it is noted that the driving ratio between the ring gear 55 and the holders 52 is such that regardless of the setting of the device 69 in either of the two positions providing for rotation of the holders, a new edge sector of each crystal occupies the leading position as the crystal face is brought into engagement with the abrasive belt during succeeding revolutions of the supporting plate 41.

If desired, the rotatable crystal holding arrangement illustrated in Fig. 8 of the drawings may be utilized in lieu of that just described. If this arrangement is employed, only the four parts ll, 42, 43 and 55 are required in the supporting structure, the pedestal member 42 itself being provided with a flat upper surface upon which the crystals may be directly supported. To this end, the crystal receiving pockets 54 are cut directly into the upper face of the member 52 to communicate with the passages 4211 which open into the bore 420 extending radially of the member 42. These pocket are of suitable configuration to receive fiatwise crystals of the particular dimensions which are to be ground, "and are of a suitable depth to permit a substantial portion of each crystal to project upwardly above the upper surface of the member 52. With this holding arrangement, shimming washers 89 may be employed to adapt the various pockets 54 for the holding of crystals [8 of different thicknesses.

Each washer utilized for this purpose is provided 7 acterized by lesser flexibility in the manner of its use, in that no facilities are provided for automatically shifting the line of cut across the crystal faces or for individually setting the crystal holders to difierent vertical positions without'the use of shimming washers. Otherwise, the manner of use of the Fig. 8 arrangement when combinedwith the essential elements of the structure shown in Fig. 3, is exactly the same as explained above.

While one embodiment of the invention has been disclosed, it will be understood that various modifications may be made therein, which are within the true spirit and scope of the invention.

I claim:

1. Apparatus for grinding the faces of piezoelectric crystals, comprising a rotatably supported plate provided with means defining crystal holding pockets which travel around a circular orbit during rotation of said plate, each of said pockets being provided with an opening through the bottom Wall thereof, vacuum producing means connected to said openings to produce suction thereat, thereby to hold the crystals in said pockets, means for rotating said plate, an endless flexible belt provided with an abrasive surface supported in a position to be successively engaged by the faces of the crystals held in said pockets during rotation of said plate, means for continuously driving said belt, and means common to said openings for breaking the suction thereat, thereby to permit the crystals to be removed from said pockets.

2. Apparatus for grinding the faces of small thin objects, comprising a rotatably supported plate provided with means defining object holding pockets which travel around a circular orbit during rotation of said plate, each of said pockets being provided with an opening through the bottom wall thereof, vacuum producing means connected to said openings to produce suction thereat, thereby to hold the objects in said pockets, means for rotating said plate, means providing an abrasive surface supported in a position to be successively engaged by the faces of the objects held in said pockets during rotation of said plate, means for continuously driving said lastnamed means, means for rotating each of the objects relative to said plate, selectively operable means for selectively rendering said last-named means active and inactive, and means common to said openings for breaking the suction thereat, thereby to permit the. objects to be removed from said pockets.

3. Apparatus for grinding the faces of small thin objects, comprising a rotatably supported plate provided with means defining object holding pockets which travel around a circular orbit during rotation of said plate, each of said pockets being provided with an opening through the bottom wall thereof, vacuum producing means connected to said openings to produce suction thereat, thereby to hold the objects in said pockets, means for rotating said plate, means providing an abrasive surface supported in a position to be successively engaged by the faces of the objects held in said pockets during rotation of said plate, means for continuously driving said last-named means, selectively operable control means for selectively rotating each of the objects relative to said plate at any one of a plurality of different speeds, and means common to said openings for breaking the suction thereat, thereby to permit the objects to be removed from said pockets.

HAL F. FRUTH. 

